Silicone rubber compositions are traditionally used in the encapsulation and sealing of electric and electronic parts for the purpose of preventing or retarding corrosion and degradation thereof. When electric and electronic parts are exposed to sulfur-containing gases such as sulfur gas and sulfur dioxide gas, conventional silicone rubbers fail to prevent or retard the sulfur-containing gas from reaching the electric and electronic parts. In particular, they are not effective for preventing or retarding corrosion of metal parts.
As modern devices comprising precision electronic parts are being reduced in size and power, challenges are made to a transition from copper to silver for the material of electrodes and chip resistors, a reduction in thickness of electrodes, and a narrowing of the gap between electrodes. Thus electronic parts become more susceptible to corrosion with sulfur-containing gas.
JP-A 2003-096301 (U.S. Ser. No. 10/252,595) and JP-A 2005-113115 disclose silicone rubber compositions for use in the encapsulation and sealing of electric or electronic parts which are obtained by adding 0.5 to 90% by weight or 0.01 to 0.5% by weight of a metal powder which is sulfidable with sulfur-containing gas to an organopolysiloxane compound. The compositions are effective for preventing or retarding the sulfur-containing gas from reaching the electric or electronic parts. The silicone rubber compositions with high loadings of metal powder, however, have the tendency that metal particles agglomerate locally, with the probable risks of short-circuiting between electrodes and reducing insulation resistance. On the other hand, the compositions with limited loadings of metal powder have the drawback that their effect does not last long. When the composition is stored for a long term, the metal powder can settle down due to a difference in density between the metal powder and the organopolysiloxane. There have been developed no silicone rubber compositions having a low viscosity enough to coat.
JP-A 2004-149611 describes that benzotriazole or derivatives thereof are added to a silicone rubber composition for restraining electric and electronic parts from corrosion with sulfur-containing gas and from electrolytic corrosion by migration. However, the amino group of these aromatic amino-bearing compounds is less active and thus less effective in restraining electric and electronic parts from corrosion with sulfur-containing gas and from electrolytic corrosion by migration.